Heat-sensitive copy sheet



Sept. l, 1964 B. L. CLARK 3,147,134

HEAT-SENSITIVE COPY SHEET Filed oct. 12, 1961 INVENTOR. BY 52x65 .CL APK United States Patent O M 3,147,134 HEAT-SENSIIIVE COPY SHEET Bryce L. Clark, St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Oct. 12, 1961, Ser. No. 144,528 4 Claims. (Cl. 117-363) This invention relates to heat-sensitive copy sheets which, while applicable to the recording of data by means of heated type, heated metal stylus, or hot air jet, are particularly useful in the copying of diiferentially radiation-absorptive graphic originals by thermographic processes involving brief intense irradiation of the original while in heat-conductive contact with the copy sheet.

Heat-sensitive copy sheets are known in which the copy is produced at heated image areas by chemical reaction resulting in reaction products which are visibly distinct from the original sheet. In some instances one or more of the reactant materials is colored, so that the copy is obtained with tinted rather than white background areas. Some reactant systems are subject to slow reaction under certain storage conditions, resulting generally in uneven or localized discoloration or in loss of sensitivity during prolonged storage. In some instances it is found ditiicult to obtain clear and sharply defined image copy on the chemically reactive type of copy paper, e.g. because of inadequate control of the irradiation or excessive variation in the graphic original.

Another form of heat-sensitive copy sheet consists of a heat-transparentizable opaque coating on a colored paper or other substrate. Localized heating of the copy sheet renders the coating transparent and permits the substrate to be seen. Such coatings are frequently soft or waxy in nature and hence are subject to mechanical displacement and disiigurement under handling or application of localized pressure, producing unwanted visible markings on the sheet. Blurred images are frequently obtained.

The present invention overcomes these and other defects and deliciencies of prior art thermo-sensitive copy sheet materials. There is produced a copy sheet which in a preferred form has an initial clean white appearance. Pressure or abrasion sufficient to compress or displace the outer coating results in no significant change in appearance. Storage under a variety of conditions does not alter the appearance or the sensitivity of the sheet. Images of improved sharpness are made possible.

These and other advantageous properties are obtained, in accordance with the present invention, by combining in a single copy sheet structure both visibly chemically reactive components and porous or particulate transparentizable opaque materials. Any color arising from the colored chemical reactants or on partial reaction during prolonged storage is to a great extent masked or hidden by the opacity of the particulate material. Premature or accidental compression or disruption or displacement ofthe opaque layer produces no significant color change at the affected areas. Images produced in the copy sheet by thermographic copying processes are surprisingly sharp and clear.

One form of the copy sheet prepared in accordance with the invention and as schematically illustrated in FIGURE 1 has a paper-like backing or base sheet provided with a lirst coating of chemically inter-reactive components in a suitable binder, and a superimposed opaque heat-transparentizable porous or particulate layer. Maximum whiteness of sheet is obtained. Localized heating of the sheet, as for example by the thermographic copying process, transparentizes the opaque outer layer and simultaneously causes reaction of the chemically inter-reactive components, with formation of strongly 3,l47,l34 Patented Sept. 1, 1964 colored reaction products which then are visible through the transparentized outer layer.

The posi-tion of the two coatings with respect to the paper-like base sheet may be reversed, i.e. to the position shown in FIGURE 3, by employing a transparent base through which the visual change in the copy layers may be observed. Additional transparent or opaque protective coatings may then advantageously be applied over the exposed reactive surface coat. The exposed transparent paper-like backing provides a desirable writing surface as well as permitting the underlying copy to be seen.

As shown in FIGURE 2, the opacifying particles may be mixed with the chemically reactive components in a single coating and with increased economy in the preparation. Unlike the inorganic pigments previously used, these particles fuse, on the application of heat, to a transparent mass through which the strongly colored reaction products are readily visible. Prior to heating, the particles add greatly to the visual opacity and whiteness of the coating, and assist in providing a white or light-colored copy sheet.

In a further construction, as represented by FIGURE 3, the chemically reactive components are applied over the porous or particulate coating and on an opaque backing. In this structure the opaque material no longer masks the reactive material, but it is unexpectedly found that image areas of unusual sharpness and clarity are formed on such a copy sheet as well as on those just previously described.

As a probable explanation for the improved sharpness and clarity of image, it is suggested that the opaque particulate or porous material serves to insulate the chemical reactants or in some other way to prevent the lateral spread of the heat-pattern around the image areas. For example the lateral flow of heat from the heated image areas may be decreased because of the insulating effect of the air pockets or voids in a porous blushed lacquer, or through heat-absorption during fusion of crystalline or waxy particulate components. In any event, it has been found that the copy sheet containing both the stratum of chemically reactive image-forming components and the stratum of porous or particulate material, either in a single layer or in two separate but adjoining layers, is capable of providing sharper and cleaner images than is obtained with either system alone.

Ordinarily it will be found desirable to employ chemical systems and transparentizable opaque materials having essentially the same conversion temperatures; and for thermographic copying processes, these temperatures will be within the approximate range of -l50 C. Sheet materials which are to be used as chart paper for marking with a heated metal stylus or with a jet of heated air, or copy sheets used in printing from heated type, may operate at higher or lower temperatures. The conversion temperature is conveniently determined by momentarily pressing the sheet against a metal test bar heated to test temperature and observing the effect on the sheet.

Copy sheets having a chemically reactive layer of rather low conversion temperature masked by a transparentizable opaque layer of higher conversion temperature produce sharp copies even though the chemically reactive layer may be reacted well beyond the initial outline of the heatpattern' supplied. Similarly, copy sheets having low conversion temperature opaque transparentizable layers applied over higher conversion temperature chemically reactive layers provide sharp images even though the outer layer may be transparentized beyond the outlines of the images formed in the reactive layer.

The opaque heat-transparentizable material may consist of separate particles of normally transparent fusible solids retained in position by small amounts of non-fusing binder. Waxes or wax-like materials are particularly useful, and

may be reduced to the desired particle size by prolonged ball milling in a solution of a resinous or polymeric binder in a volatile liquid vehicle which is a non-solvent for the fusible solid. Soluble crystalline solids are also useful, the composition being so formulated as to permit crystallization of the solid in iinely particulate form during drying of the coating. A preferred structure employs a blushed lacquer as both the porous or particulate material and the binder material. Plasticizers and softeners may be added as needed, e.g., in order to control the conversion temperature. The blushed lacquer forms an interconnected network of very small particles which, like the layer of wax particles, is opaque primarily because of the presence of voids or light-refracting interfacial surfaces. The structure is commonly produced by deposition from a volatile vehicle containing both solvent and non-solvent for the hlm-forming resin or polymer composition, the ratio determining the degree of porosity of the deposited coating. Combinations of blushed lacquer and waxy or crystalline particulate solids are also useful, the particles being bonded by the lacquer coating and adding to its initial opacity as well as its ability to be transparentized on heating. Particulate solids and resins or polymers which form compatible mixtures are particularly desirable since the transparent blend obtained on fusion remains permanently transparent after cooling. Nonfusing particles, eg. glass particles or other inorganic fillers, may be added where desired for special purposes.

For the chemically reactive components there may be used any of a wide variety of reactant systems providing normally stable coatings which undergo distinct visible change or color formation on momentary heating. The reactants may be in closely intermingled particulate form in a non-fusing binder layer, being brought together into inter-reactive association on fusion of one or more components of the coating as the sheet is momentarily heated; or transmission of vapor within the coating may precede the color-forming reaction; or some other mechanism may be involved. The reactants may less desirably be contained in separate but consecutive coatings or layers, reaction then being initiated at the interface on the application of heat. The amounts and proportions of reactants and of other components must be so selected as to make certain that inter-reaction may occur and to an extent sufiicient to produce a distinctly visible change, but stoichiometric proportions are not essential and in many instances may be undesirable.

The invention will now be further illustrated by means of specific working examples which however are not to be construed as limitative. All proportions are given in parts by weight unless otherwise stated.

Example 1 Ferrie tristearate is dispersed in acetone, by prolonged ball milling, at a concentration of 35%. Hexamethylene tetramine gallate, formed by precipitation from a mixture of alcohol solutions of three parts of hexamethylene tetramine and four parts of gallic acid, is similarly dispersed in acetone at a concentration of 40%. Five parts of the rst dispersion and 15 parts of the second are mixed with tive parts of a 10% solution of ethyl cellulose in acetone. The mixture is coated on white paper using a coating bar set at an orifice of 5 mils (.005 inch), and dried in a current of air at room temperature. The coating is a light buff in color. Placed in contact with a metal test bar at a temperature of 100 C., the coating rapidly becomes a dense blue-black at the contacted area.

A second coating is applied at the same coating thickness over the chemically reactive coating and dried as before. The second coating is prepared by mixing together 30 parts of a solution of l5 parts of cellulose acetate in 56 parts of acetone and 29 parts of toluene, and 5 parts of diethyl phthalate plasticizer. The dried blushed lacquer coating has a clean white appearance. Contact with the heated test bar results in the transparentization of the outer white coating and darkening of the inner chemically reactive coating to produce a dense blueblack image area against a contrasting white background.

The sheet is useful in the copying of printed originals by the thermographic copying process. The image areas are high in contrast and are clear and sharp in appearance.

Example 2 A mixture of five parts of the ferrie tristearate dispersion, 15 parts of the hexamethylene tetramine gallate dispersion, 30 parts of the 15% cellulose acetate solution, and live parts of diethyl phthalate, all as provided in Example 1, is coated on white paper at a coating orifice of 2 mils, and dried to provide an off-white heat-sensitive copy-sheet which is less brilliantly white in appearance than the copy-sheet of Example 1 but is significantly less colored than the intermediate buff colored sheet of that example. The copy-sheet is converted to a dense blueblack by brief contact with the heated test bar or in the thermographic copying process.

Example 3 A first coat is provided on white paper by dipping the paper through a solution of 10 parts of cobaltous acetate and 20 parts of thiourea in 70 parts of methyl alcohol. The sheet is dried and is then coated at a thickness of three mils with a solution of 15 parts cellulose acetate and 3 parts diethyl phthalate in 56 parts acetone and 29 parts toluene. The resulting dried copy-sheet has a white glossy surface which is converted to a dense black appearance on contact with a hot stylus at 150 C. or on localized heating in the thermographic copying process.

Example 4 Eight parts of a mixture of 15 parts of nickel acetate and 30 parts of ammonium thiosulfate in 100 parts of water is mixed with 40 parts of a solution of 10 parts of ethyl cellulose in parts of acetone. The mixture is coated at a wet thickness of 4 mils on White paper, and dried. The white cop-sheet blackens when briey heated at C. either by contact with a heated test bar or in the thermographic copying process.

xample 5 A first dispersion is prepared from the following components by milling them together in a ball mill:

Parts Polyvinyl alcohol 6.05 Zinc oxide 12.10 Silver behenate-behenic acid 7.95

Phthalazinone 1.21

Water 63.50 Acetone 9.07

Citric acid 0.15

The silver behenate mixture is prepared by reacting together one mol of silver nitrate and two mols of sodium behenate in an aqueous medium and in the presence of nitric acid. The washed and dried water-insoluble precipitate fuses at about C. and melts to a liquid at about C.

Separately there is prepared a solution of the following components by mixing:

Parts Methyl gallate 5.16 Tetrachlorophthalic anhydride 0.17 Acetone 8.60

Water 77.40 Polyvinyl alcohol 8.60

A mixture of 2.84 parts of the dispersion and one part of the solution is coated on white paper at a coating orifice of 3 mils and dried to produce a white coating which is stable under controlled conditions but shows a tendency to darken when stored at high humidities and at moderately elevated temperatures. The coating is converted to a dense brownish black when briefly heated to about 120 C. by contact with a heated test bar or in the thermographic copying process.

The sheet is further coated with a thin coating of a blushed lacquer as provided in Example 1, producing a white surface appearance which remains unchanged on storing the sheet at high humidities and moderately elevated temperatures. The surface coating is transparentized and the sub-coating blackened to produce a clear and sharp image area when the sheet is locally, briefly heated to the conversion temperature.

Example 6 Another opacifying coat which may be applied over the color-forming coating of Example 1 or 5 is prepared by mixing together 30 parts of a solution of 15 parts cellulose acetate in 56 parts acetone and 29I parts toluene with a mixture of 3 parts amorphous silica powder, 3 parts triphenyl phosphate, 5 parts dichlorohexyl phthalate, and parts acetone. On drying, the coating provides a white appearance and is transparentized on brief heating to 130- 140 C.

Example 7 In place of the coating composition of Example 6 there may be used a mixture of 30 grams of a solution of l5 parts nitrocellulose in 47.5 parts ethyl acetate and 47.5 parts xylol with a solution of 3 parts acetyl salicylic acid in live parts acetone. The dried coating is white and opaque, and converts to a permanently transparent state when briefly heated to l10120 C.

Example 8 A further` example of an opaque heat transparentizable coating composition for use in conjunction with a chemically reactive coating or composition as herein previously described is prepared by mixing together 30 parts of the nitro-cellulose solution of Example 7 and a solution of three parts diphenyl phthalate in 10 parts of acetone and live parts of xylol. The opaque dry coating becomes permanently transparent on being heated to 7075 C.

Example 9 A transparent thin (2-mil) film of Mylar tensilized polyethylene terephthalate polyester is coated With a thin uniform layer of a solution containing ten grams of Lucite 41 methacrylate polymer, 100 ml. of acetone, 50 ml. of butyl alcohol, and 0.5 ml. of butyl stearate. The dried coating is white and opaque. It transparentizes on contact with a metal test bar at 90-l00 C. or under localized heating in the thermographic copying process, providing permanently transparent windows in the opaque layer.

A chemically reactive heat-sensitive coating is next applied over the opaque surface. It conists of a mixture of ten parts of a 10% suspension of silver behenate in a 10% solution of Pliolite VT polymerized vinyl toluene in commercial heptane, and ve parts of a 5% suspension of methyl gallate in the 10% polyvinyltoluene solution. The coated and dried sheet is white and opaque in appearance and has an over-all thickness of about 6 mils.

As a control sheet, a portion of the polyester ilm is directly coated With the same thickness of the silver behenate/methyl gallate/ binder suspension. The two sheets are tested for image resolution by making a copy on each of a photographic test chart original printed with groups of narrow parallel lines at progressively reduced spacing. Copying is accomplished by the thermographic frontprinting process with the coated surface in exposed position. Visual inspection shows that the doubly coated sheet has signicantly higher resolving power than does the control sheet.

What is claimed is as follows:

l. A heat-sensitive copy sheet useful in making highresolution copies of differentially radiation-absorptive graphic originals by a thermographic process involving brief intense irradiation of said original while in heatconductive contact with said copy sheet and including, in combination, a stratum of chemically inter-reactive components in uniform intimate juxtaposition in sulcient amount and in condition for chemically reacting to form a visibly differently colored reaction product on briellyv heating the copy sheet, and a particulate opaque heattransparentizable stratum associated with and at least partially concealing said reacting stratum and adapted to be rendered permanently transparent on briey heating the copy sheet.

2. A heat-sensitive copy sheet including, in order, a paper-like backing, a chemically reactive coating capable on brief heating to a conversion temperature within the approximate range of -150 C. of converting to a permanently visibly distinct appearance by chemical reaction, and an opaque particulate coating overlying and obscuring said reactive coating and being permanently transparentizable on brief heating to a conversion temperature within the approximate range of 90-150 C.

3. A heat-sensitive copy sheet having a paper-like backing and a heat-sensitive coating thereon comprising a mixture of chemically inter-reactive components and particulate fusible hlm-forming material, said particulate materials at least partially concealing said inter-reactive components and being transparentizable by fusion on briefly heating at a conversion temperature Within the approximate range of 90-150" C., said inter-active components being capable of converting by chemical interreaction to a permanently visibly distinct appearance on briefly heating at said conversion temperature.

4. A heat-sensitive copy sheet having a paper-like backing, an opaque heat-transparentizable particulate subcoating, and a visibly heat-sensitive chemically reactive coating capable on brief heating to a conversion temperature within the approximate range of 90-l50 C. of converting to a permanently visibly distinct appearance by chemical reaction.

References Cited in the tile of this patent UNITED STATES PATENTS 2,519,660 .lames Aug. 22, 1950 2,730,457 Green et al Jan. 10, 1956 2,807,545 Frederick Sept. 24, 1957 2,940,866 Sprague et al June 14, 1960 2,967,784 Newman et al. Ian. 10, 1961 3,057,999 Newman et al. Oct. 9, 1962 

1. A HEAT-SENSITIVE COPY SHEET USEFUL IN MAKING HIGHRESOLUTION COPIES OF DIFFERENTIALLY RADIATION-ABSORPTIVE GRAPHIC ORIGINALS BY A THERMOGRAPHIC PROCESS INVOLVING BRIEF INTENSE IRRADIATION OF SAID ORIGINAL WHILE IN HEATCONDUCTIVE CONTACT WITH SAID COPY SHEET AND INCLUDING, IN COMBINATION, A STRATUM OF CHEMICALLY INTER-REACTIVE COMPONENTS IN UNIFORM INTIMATE JUXTAPOSITION IN SUFFICIENT AMOUNT AND IN CONDITION FOR CHEMICALLY REACTING TO FORM A VISIBLY DIFFERENTLY COLORED REACTION PRODUCT ON BRIEFLY HEATING THE COPY SHEET, AND A PARTICULATE OPAQUE HEATTRANSPARENTIZABLE STRATUM ASSOCIATED WITH AND AT LEAST PARTIALLY CONCEALING SAID REACTING STRATUM AND ADAPTED TO BE RENDERED PERMANENTLY TRANSPARENT ON BRIEFLY HEATING THE COPY SHEET. 